This invention applies to pivotable closure panels on vehicles, in particular to closure panels that rotate about their lower edge and require significant effort on the part of the operator to open and close, such as vehicle tailgates.
Although it is common practice not to provide vehicle tailgates with lift assistance, there have been numerous devices suggested in the prior art that are configured to do so. With the continued sophistication of vehicle systems and the addition of comfort and convenience items, tailgates are becoming increasingly heavier and lift assistance devices will become a requirement. The most common form of tailgate assist outlined in the prior art uses a torsion spring packaged with the tailgate that stores energy in response to opening angle.
U.S. Pat. No. 5,358,301 issued to Konchan et al discloses a torque rod counterbalance spring carried in the tailgate panel that is torsionally wound when the panel is pivoted away from the panel removal position. The torque rod is straight and oriented coaxially with the tailgate pivot brackets. The energy stored in the torque rod assists the operator in lifting the tailgate up to the removal position. The tailgate panel can be removed from the vehicle in the removal position as two features on the pivot bracket are aligned. Unfortunately, the system described in Konchan adds significant weight to the overall tailgate mass and therefore makes the assembly difficult to manage when removed.
U.S. Pat. No. 5,988,724 issued to Wolda discloses a similar mechanism to Konchan et al but utilizing a horseshoe-shaped torque rod that can be rapidly installed as an option. Other than the capability of having the torque rod independent of the pivot brackets and therefore capable of optional usage, the device described in Wolda is similar to that of Konchan et al and suffers the same weight penalty over an unassisted panel.
There is another large class of lift assist systems that are configured to be mounted to a body structure and act upon the pivotable closure panel with a direct force. These types of devices are utilized extensively on closure panels that pivot at their upper edge such as vehicle liftgates for vans and the like. These devices would also be suitable for usage on a tailgate but would require large packaging volume due to their requirement to act on the closure panel at a distance from the hinge pivot bracket axis. Additionally, because they mount to the body structure, they would hinder the removal of the tailgate from the vehicle. Unlike vehicle liftgates, simple removal of tailgates is a requirement of most tailgate systems.
U.S. Pat. No. 4,291,501 issued to Steniberg et al discloses a device for counterbalancing a swiveable body part or closure panel on a motor vehicle using a torsion bar with integrated crank. The torsion bar is packaged externally from the closure panel and is not coaxial to the pivot bracket axis therefore creating a large packaging volume requirement.
In addition to torsion bars, a wide range of other energy storage devices are utilized to generate direct forces on closure panels in a similar manner to Steniberg et al. These devices include, but are not limited to, extension springs, compression springs, clock springs and gas springs.
Accordingly, it would be advantageous to provide a simple lift assist system for a vehicle tailgate that is easily disengaged and does not substantially increase the weight of the tailgate, so that removal of the tailgate from the vehicle is unimpaired. Additionally, is would be advantageous to arrange the lift assist system to impart a torque and orient it coaxially with the tailgate pivot axis so that packaging volume is minimized and the relatively simple removal process of a conventional prior art tailgate is maintained.
The present invention provides a lift assist system for a vehicle tailgate utilizing an energy storage device packaged in the vehicle structure. The energy storage device is configured so as to impart a torque on an actuation shaft via a bellcrank. The actuation shaft is placed in coaxial alignment with the tailgate pivot bracket axis and is configured with a slot drive feature that couples it to the tailgate in cooperation with a conventional prior art pivot bracket assembly. The slot drive feature automatically decouples without operator intervention when the tailgate is removed in a conventional manner as known in the prior art. The slot drive feature, bellcrank and actuation shaft are arranged so as to impart an increasing lift assist torque on the tailgate in response to pivotally opening the tailgate. In the preferred embodiment of the present invention the slot drive arrangement is configured to disengage the lift assist torque at an intermediate position of the tailgate that coincides with the removal position dictated by a conventional prior art pivot bracket assembly. In this manner the lift assist stop and tailgate removal position occur at the same position which significantly simplifies the removal and reinstallation process. Additionally the energy storage device can be configured to incorporate viscous damping in the compression direction so that the opening velocity of the tailgate can be limited.
Accordingly, in a principal aspect of the invention, a vehicle tailgate lift assist system comprises a vehicle tailgate adapted to open and close by rotating about a tailgate pivot axis in relation to a vehicle body, an energy storage device located within the vehicle body and adapted to be pivotally anchored to the vehicle body at one end, a bellcrank arm pivotally attached to the energy storage device and rigidly attached to an actuation shaft that is coaxially aligned with the tailgate pivot axis, a pivot bracket assembly comprising a pivot bearing arrangement to facilitate a robust rotary joint about the tailgate pivot axis, and comprising a coupling means between the tailgate and the actuation shaft, such that relative rotary motion of the tailgate and the vehicle body results in linear movement of the energy storage device and maximum energy storage at the tailgate fully open position.
In further aspects of the invention:
a) the energy storage device is a gas spring;
b) the gas spring incorporates viscous damping in the compression direction to limit the opening velocity of the tailgate;
c) the energy storage device is a coil spring, elastomer or similar resilient material;
d) the coupling means between the tailgate and actuation shaft comprises a slot drive arrangement that can be simply decoupled to allow the removal of the tailgate;
e) the slot drive arrangement is configured to disengage the closing assist torque at an intermediate position of the tailgate which is configured to coincide with the removal position of a tailgate pivot bracket system.
Further aspects of the invention will become apparent from the following description.